Physiological maturity as a function of seed and pod water concentration in spring rapeseed (Brassica napus L.)

Abstract

Determining the optimum time for rapeseed harvest is challenging due to non-uniform seed maturity resulting from asynchronous flowering and pod dehiscence from sequential racemes. Identifying physiological maturity (PM) by visual methods is subjective and results can be affected by environmental conditions. PM can be determined using a quantitative model based on seed water concentration (SWC) as previously demonstrated for several other crops, although not yet developed for rapeseed crop. The objective of this work was to study the relationship between the dynamics of seed dry weight and water concentration in seven spring rapeseed cultivars grown at two contrasting densities (15 and 60 pl m−2) in three experiments at one location in Buenos Aires (Argentina). We evaluated the timing of PM on the basis of SWC in seeds located in the main raceme, second and fourth floral branches. The evolution of seed fresh and dry weight was followed bi-weekly from the beginning of flowering to harvest maturity. In Exp. 1, the grain-filling duration ranged from 39 to 57 days (700–1100 °C d) and the growth of seeds from floral branches finished 3–8 days later than those from the main raceme. Seed dry weight at PM ranged from 2.4 to 2.7 and from 3.0 to 3.2 mg for Lynx and Monty cultivars, respectively, without significant effects of floral position or plant density. Bi-linear functions were used to fit the relationship between relative seed dry weight (RSDW) and SWC relationships (R2 from 0.85 to 0.95). Across cultivars and floral positions, PM was attained when seeds exhibited 46.3 ± 0.7% SWC (R2 = 0.90, P < 0.001, n = 441). This model was validated against independent data from Exps. 2 and 3, successfully simulating the dynamics of relative seed dry weight based on fruit WC (r = 0.88; P < 0.001, n = 275). At PM, the water content (WC) of whole pod was about 70% and the pod shattering began after this point, when the WC of the pod dropped drastically. We conclude that under non-stressful conditions, PM in rapeseed occurs at 46% SWC. Swathing can be conducted from SWC < 46%, instead of the currently recommended 35%, advancing the harvest and leaving the land available for sowing the next crop, which would represent an advantage for double cropping in intensified agricultural systems.